MADRID -- Merrill Lynch & Co. said Claudio Aguirre will become head of investment banking for Europe, Africa, and the Middle East, effective March 4. Mr. Aguirre, who becomes the dean of Spanish investment bankers in Europe, will oversee mergers and acquisitions and capital market activities for the region.

Mr. Aguirre replaces Justin Dowley, 45 years old, and Huston McCollough, 49. Mr. Dowley is leaving to pursue outside interests and will be a senior advisor to Merrill Lynch. Mr. McCollough has requested a sabbatical...